Project Summary (Abstract): Graft-versus-host disease (GvHD) remains the most common cause of non-relapse mortality post hematopoietic cell transplant (HCT). Up to 40% of HCT recipients will fail first-line immunoprophylaxis and ultimately develop GvHD. Standard-of-care strategies for GvHD have not changed in decades and new therapies are desperately needed. Our team is currently enrolling patients in a Phase II clinical trial of Abatacept, a T cell costimulatory inhibitor as an adjunct to standard GvHD immunoprophylaxis, which is currently supported by an R01 mechanism under the Clinical Studies of Safety and Effectiveness of Orphan Products Program. In this Competing Supplement application, we propose to perform a critical correlative biology study that is expected to provide insights into the mechanisms that drive breakthrough GVHD. In the proposed study, we will perform a retrospective, case-matched, correlative biology study with samples from the Abatacept Phase II (ABA-02) clinical trial biorepository. Using next-generation sequencing technology, we will identify the mechanism of breakthrough GvHD in patients being treated on the ABA-02 study. We will also sequence patient samples prior to the onset of clinical GvHD, in order to identify gene signatures associated with sub-clinical GvHD. These studies will provide mechanistic insights into the factors that control breakthrough GvHD, and lay the foundation for a risk-stratified approach to GvHD prevention.